Control Effects Of Gypsum Rock Upon Pre-bsalt Hydrocarbon Accumulation In Santos Basin

The Albian Ariri Formation in the Santos Basin contains rich petroleum resources during the Cretaceous subsalt of the gypsum-salt rock.So far,more than 55 billion barrels of proven oil and gas reserves have been submitted,which shows the huge exploration potential of the gypsum-salt rock in the Santos Basin.However,the controlling effect of gypsum-salt rocks on the formation of oil and gas below the study area is not clear.Based on the study of the structure and stratigraphy of the Santos Basin,this paper extensively collects public literature data,uses the seismic,drilling,and logging data of the IHS commercial database,as well as previous research results,to analyze the sub-salt accumulation conditions,including the characteristics of source rocks,reservoir characteristics,caprock characteristics and gypsum salt rock development characteristics.Understand the distribution of oil and gas in the Santos Basin,and use the comprehensive research of petroleum geology and Petro Mod software to explore the control effect of the gypsum-salt rock on the migration and accumulation of oil and gas in the Santos Basin.It is conducive to the understanding of sub-salt oil and gas reservoirs,and further expands the exploration areas of deep-water salt basins similar to the Santos Basin in Brazil.Research indicates:1.The gypsum salt rock has the characteristics of strong plasticity and easy flow.The gypsum-salt rocks of the Ariri Formation of the Apte Stage in the Santos Basin are widely distributed,with obvious differences in the horizontal and vertical distribution.The central low convex belt and the offshore low concave belt are the discontinuous distribution area of gypsum salt rock,and the outer high area is the continuous distribution area of salt rock and the thick salt rock area.2.Basin simulation results show that the maturity of the source rock in the study area is mainly for oil generation in offshore depressions and nearshore depressions,with large local burial depths and thinner gypsum salt rocks.The thermal evolution maturity of source rocks is higher.It can reach the stage of wet gas production,the maturity of the outer high zone is low,and it is in the early stage of oil generation-oil generation stage.3.The oil and gas in the nearshore depression zone migrates to the diagonal belt on both sides.High-permeability reservoirs on the migration path can form lithological oil and gas reservoirs.The central low convex belt accepts hydrocarbon supply from the depressions on both sides,and part of the oil and gas migrate along the fault to the upper salt horizon to accumulate into reservoirs.The offshore depressions are the main hydrocarbon-generating depressions.The generated oil and gas are discharged to both sides of the bulge.Part of the oil and gas migrates along the fault to the upper-salt horizon to accumulate and accumulate,while the other part of the oil and gas accumulates in the sub-salt horizon.The oil and gas in the outer high zone comes from its own hydrocarbon generation and the adjacent offshore low depression zone,and the oil and gas are distributed in the high part of the structure or the stratigraphic-structural trap along the migration path.4.The gypsum salt rock with high thermal conductivity leads to a 12% decrease in the temperature of the sub-salt area and an increase of 9% in the supersalt area.The presence of gypsum salt delays the evolution of organic matter in the source rock,which is conducive to the preservation of organic matter and hydrocarbon generation.Thick layers in the Santos Basin Subsalt source rocks are still in the oil generation window.5.The controlling effect of gypsum-salt rock on its underlying hydrocarbon accumulation is mainly reflected in two aspects.On the one hand,the gypsum-salt rock layer makes the subsalt ground temperature abnormal and delays the oil generation stage of the source rock in the thick gypsum-salt rock;On the one hand,the salt window developed in the central low convex belt and the northwestern offshore depression provides a migration channel for the upward migration of sub-salt oil and gas,which mainly accumulates in the upper salt strata;the southeast of the offshore depression belt and the outer high zone are continuous thick layers.The gypsum salt rock acts as a caprock to control the accumulation of sub-salt oil and gas.